The goal of the proposed studies is to understand which molecules are responsible for the differences between male and female brains. It is well known that testosterone, secreted by the testes before or after birth, acts on the male brain to masculinize specific neural networks, which results in specific masculine behavior. The classical view is that gonadal androgens are the only factors involved in the masculinization of the brain. We propose to explore an alternative possibility. Sex differences in behavior may be caused in part by genetic factors influencing the development of the brain. We will search for the genes responsible for brain sexual dimorphims by determining which genes are differentially expressed between male and female brains, and where in the brain these genes are expressed (Specific Aims 1 and 2). We will characterize the profile of expression of these genes in order to test their relevance to the development of brain sexual dimorphisms (Specific Aim 3). The ultimate goal of this project is to create mouse models in which female mice express male genes only in their brain, in order to elucidate the role of these genes in sexual behavior. This proposal intends to improve the general understanding of the sexual differentiation of the brain. It will also impact significantly our knowledge of a number of neurological and psychiatric disorders where sex differences are observed, such as Alzheimer's disease, schizophrenia, and depression.